Non-chromate sealant for porous anodized aluminum

ABSTRACT

The present invention provides a method for easily and effectively removing adsorbed water molecules from an anodized surface using low intensity ultraviolet (UV) radiation. The present invention also provides a method for sealing an anodized aluminum surface which does not result in hazardous byproducts. The method involves, in vacuum: (1) vaporizing a selected precursor fluid; (2) condensing a flux of said precursor vapor onto the anodized aluminum surface; (3) and, bombarding said condensed precursor vapor with an energetic beam of ions to convert the porous anodized surface into an inert, solid, impermeable, and mechanically strong surface.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.08/400,612, filed on Mar. 8, 1995, abandoned.

FIELD OF THE INVENTION

The present invention relates to an improved method for sealing theporous surface of anodized aluminum and its alloys. The filling medium,which is chemically inert and impermeable, forms a mechanically strongamorphous or diamond like carbon coating that will withstand exposure tohigh temperatures. The process results in byproducts that are lesshazardous to the environment than those produced by chromate solutions.

BACKGROUND OF THE INVENTION

Aluminum is commonly used to manufacture many different articles. Whencompared to steel, aluminum owes its versatility as an engineeringmaterial to its easy workability, its somewhat low specific gravity, andits relative resistance to corrosion by the ambient environment.

The resistance to corrosion exhibited by aluminum is due to theformation of a substantially transparent "natural" oxide layer uponexposure to air. Unfortunately, this "natural oxide" layer does notalways have a uniform thickness. Because of this, natural oxidesgenerally are removed from aluminum products, and the product thereafteris "anodized," or controllably oxidized, to provide a protective oxidelayer with better quality.

Anodizing processes generally involve the use of a bath containing anelectrolyte, such as sulfuric acid, oxalic acid, chromic acid,phosphoric acid, or combinations thereof, with or without certainaddition agents. The aluminum workpiece generally is used as an anodeand a component made of steel or other suitable material is used as acathode. The anode and cathode are immersed in the electrolyte solution,and a direct or alternating current is passed through the electrolyte.

Although anodizing, itself, imparts satisfactory corrosion resistance toaluminum components, anodizing also suffers from several disadvantages.One disadvantage is the porosity of the oxide formed at the surface ofthe aluminum component. A typical anodizing treatment results in aporous polygonal cellular microstructure superimposed on a thin (lessthan 100 nm) "barrier" layer. The diameter of the pores in themicrostructure can be as small as 10 nm. The cell dimension can be assmall as about 30 nm.

The pores formed at the surface of anodized aluminum are undesirablebecause they tend to serve as corrosion sites, which give rise to deeppits. Deep pits in the anodized surface often result in "blooms" orwhite spots on the surface of the aluminum. In order to protect anodizedaluminum from corrosion, especially in halide or salt-containingenvironments, the pores of the aluminum oxide customarily are sealed byimmersion in a hot solution containing hexavalent chromium. A complexchemical reaction occurs, forming a solid compound of chromium,aluminum, oxygen, and some hydrogen within the pores of the anodizedsurface. This solid compound seals the pores against penetration bycorrosive agents.

Unfortunately, hexavalent chromium solutions are toxic. The use anddisposal of hexavalent chromium solutions therefore createsenvironmental concerns. These concerns, and their associated costs, havecreated an urgent need for an alternative sealing process that is freefrom such hazards.

Some have attempted to develop alternative sealing processes using otherchemical solutions. To date, these alternative chemical solutions havenot been entirely successful. A non-toxic, effective method for sealinganodized aluminum surfaces is urgently needed.

Most anodizing treatments require that the aluminum component beimmersed in an aqueous solution. Even after drying, a film of watermolecules (about two monolayers thick) tends to remain strongly adsorbedto the anodized surface. Where the anodized surface will be treated witha relatively hydrophilic sealant, the presence of such adsorbed watermolecules should not interfere with the sealing process. However, if theanodized surface will be treated with a hydrophobic sealant, theadsorbed water molecules could interfere with the sealing process, andshould be removed from the surface before the sealant is applied.

The removal of water molecules from an anodized surface is not a simplematter. Water molecules are polar, and thus have a charge distributionwithin the molecules, themselves. The attraction between the anodizedsurface and the polarized water molecules creates a weak bond whichholds the water molecules to the anodized surface. In order to breakthis weak bond, the water molecules must be provided with enough energyto break free from the anodized surface.

A number of methods exist for freeing adsorbed water molecules fromvarious surfaces. These methods include exposing the anodized surfaceto: sonar energy; heat; a flow of inert gas; a beam of de-focusedelectrons; and, UV light.

The use of sonar energy to free adsorbed water molecules has proven tobe time consuming and not entirely successful. Heating of the surface ismore successful in actually desorbing the water molecules from thesurface; however, not all of the adsorbed water molecules are removed byheat, and the application of heat can be cumbersome and time consuming.A flow of inert gas, such as nitrogen, removes some adsorbed watermolecules; however, the movement of the gas molecules is random, and itis likely that not all of the adsorbed water molecules will be removedby the gas. Whether de-focused electrons can successfully removeadsorbed water molecules from an anodized surface is not known; however,the technique has not been used commercially.

Water molecules absorb certain wavelengths of UV light. The absorbedenergy should excite the water molecules into a vibrational mode,freeing the water molecules from the surface to which they are adsorbed.However, the UV light that has been used in the past to desorb watermolecules from various surfaces has been relatively high intensity, orshort wavelength UV light. The conventional source of UV light is amercury vapor lamp. In most mercury vapor lamps, essentially allradiation having a wavelength shorter than 200 nm is shut off by asilica envelope. Water has a low coefficient of absorption in therelatively short wavelength ranges produced by mercury vapor UV lamps.As a result, a relatively long period of time has been required todesorb water molecules from a surface using short wavelength UV light.

A more effective and economic method is needed for removing adsorbedwater molecules from anodized surfaces. Also needed is a method forsealing an anodized aluminum surface with a medium that is chemicallyinert and impermeable, using a process that results in byproducts thatare less hazardous to the environment than hexavalent chromium.

SUMMARY OF THE INVENTION

The present invention provides a method for easily and effectivelyremoving adsorbed water molecules from an anodized aluminum surfaceusing low intensity ultraviolet (UV) radiation. The present inventionalso provides a method for sealing an anodized aluminum surface withoutproducing hazardous byproducts. The method involves, in vacuum: (1)vaporizing a selected diamond-like or amorphous carbon precursor fluid;(2) condensing a flux of the diamond-like or amorphous carbon precursorvapor onto the anodized aluminum surface; (3) and, bombarding thecondensed precursor vapor with an energetic beam of ions to convert theporous anodized surface into an inert, solid, impermeable, andmechanically strong amorphous or diamond like carbon coating.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, "aluminum" shall mean aluminum and alloys thereof thatare amenable to anodization. The sealing process of the presentinvention involves the application of a nonaqueous, relativelyhydrophobic precursor fluid to an anodized aluminum surface. Thepresence of water molecules adsorbed to the anodized surface most likelywould interfere with the application of the hydrophobic precursor fluid.Therefore, a method is provided for effectively removing adsorbed watermolecules from the anodized surface before depositing the precursorfluid.

Water molecules have a much higher coefficient of absorption for UVlight with a longer wavelength, in the region of 120-150 nm, than forthe short wavelength UV light produced by conventional UV lamps.Exposure of adsorbed water molecules to low intensity UV light shouldresult in more rapid, and more effective desorption of the watermolecules from the anodized surface.

Longer wavelength UV radiation can be obtained using unconventional UVlamps, such as deuterium discharge lamps. Deuterium discharge lampsgenerate UV radiation having wavelengths down to 120 nm. These lowerwavelength UV lamps can be modified, using special windows formed ofsubstances such as magnesium fluoride, to transmit radiation down towavelengths of about 110 nm.

To treat an anodized aluminum component, the component should placed ina vacuum chamber provided with: (a) a source of low intensity UVradiation; (b) a reservoir for vaporizing the precursor sealant fluidand directing the vapor onto the component; and (c) an ion gun or othersuitable apparatus for accelerating ions and bombarding the componentwith an energetic beam of ions.

The pressure in the vacuum chamber should be pumped down to at leastabout 10⁻⁶ torr. In a preferred embodiment, a 150 watt UV lamp is usedto produce UV radiation in the range of about 110-180 nm, preferablybetween about 120-150 nm. The surface of the anodized aluminum should beexposed to a flux of this low intensity UV radiation for a timesufficient to remove adsorbed water molecules from the anodized surface.Using a 150 watt lamp and 120-150 nm UV light, this should take about 20minutes.

In a preferred embodiment, the reservoir is supplied with electricalresistance heating. The reservoir should contain a selected amorphous ordiamond like carbon precursor fluid in an amount sufficient tovolatilize and coat the component. A number of precursor materials foramorphous or diamond-like carbon coatings are known in the art, and anyof the known precursor materials would be suitable for use in thepresent invention. Suitable precursor materials include diffusion pumpmaterials which have a low vapor pressure and can be vaporized stably atroom temperature. Preferable diffusion pump fluids include polyphenylether, polydimethyl siloxane, pentaphenyltrimethyl siloxane, and elcosylnapthalene. Persons of ordinary skill in the art will recognize that"diffusion pump fluids" are a discrete group of vacuum distilled,carbon-containing mineral oils, such as the Apiezon group, or syntheticoils, particularly the higher-order esters such as the phthalates andsebacates, or silicones and chlorinated hydrocarbon oils. Diffusion pumpfluids are used to create a vacuum in a vacuum chamber using a diffusionpump. Diffusion pump fluids have the following common features: (a) theycontain carbon; (b) they must not have substantial vapor pressure, whichcould increase the pressure in the vacuum chamber; and, (c) they must beable to vaporize thermally and condense onto a cooled surface withoutdecomposing in the process.

Other carbon-containing materials besides diffusion pump fluids that aresuitable for use as precursor materials include fullerenes (described inU.S. Pat. No. 5,393,572, incorporated herein by reference) and parylenes(described in U.S. Pat. No. 5,512,330, incorporated herein byreference). Preferably, the reservoir should be heated to an appropriatetemperature to vaporize the selected precursor, and the resulting vaporflux should be directed through an aperture or nozzle to direct the fluxtoward the surface to be sealed until a preferred coating thickness ofbetween about 1-5μ is achieved. The thickness of the coating may bemonitored by standard methods, e.g., using the frequency charge of aquartz crystal oscillator.

At the same time, the component should be bombarded, either in acontinuous or interrupted fashion, with an energetic beam of ions. Abeam of substantially any energetic ions should function in the presentinvention. Preferable ions are preferably ionized gaseous species suchas hydrogen, helium, neon, nitrogen, argon, methane, carbon monoxide, orother relatively low mass gaseous elements or compounds. The energy ofbombardment must be sufficient to ionize the constituent molecules inthe precursor film, and to rupture the bonds between hydrogen and otheratoms, such as carbon and silicon, thereby releasing the hydrogen intothe surrounding vacuum to be pumped away, leaving an amorphous ordiamond-like carbon coating. The energy of bombardment can range frombetween about 1 keV to about 1 MeV, but preferably should be betweenabout 20 keV to about 100 keV.

The rate of arrival of the ions should be controlled in relation to therate of arrival of the precursor molecules. This process should requireabout one ion for every 100 atoms in the final product coating; however,the ion-to-atom ratio will vary according to the mass and energy of theion species. Persons skilled in the art will recognize how to achievethe correct linear energy transfer in the ionizing process.

The ion bombardment should be continued until the precursor moleculesare ionized and converted into an inert, solid, impermeable, andmechanically strong amorphous or diamond-like carbon coating. The amountof time required to achieve this conversion will vary with the intensityof the ion beam. At an ion-to-atom ratio of 1 to 100 and an energy ofabout 20 keV to about 100 keV, about 30 minutes of ion bombardmentshould be sufficient. Depending upon the chemical nature of theprecursor, the resulting surface should be carbonaceous, silicaceous, ora blend of carbon and silicon product, with some residual hydrogenand--if oxygen was present in the precursor--residual oxygen.

Persons of skill in the art will appreciate that many modifications maybe made to the embodiments described herein without departing from thespirit of the present invention. Accordingly, the embodiments describedherein are illustrative only and are not intended to limit the scope ofthe present invention.

We claim:
 1. A method for sealing a porous anodized aluminum surfacecomprising:placing a component having an anodized aluminum surface in avacuum chamber evacuated to a pressure of about 10⁻⁶ torr; condensingonto said surface a precursor material in an amount sufficient, upon ionbombardment, to form an inert, substantially impermeable amorphouscarbonaceous seal; substantially simultaneously bombarding said anodizedsurface with an energetic beam of ions at an energy between about 1 keVto about 1 Mev for a time and at a linear energy of transfer sufficientto convert said precursor material into said inert, substantiallyimpermeable amorphous carbonaceous seal.
 2. The method of claim 1wherein said energy of ion bombardment is between about 20-100 keV. 3.The method of claim 2 wherein, before condensing said precursor materialonto said surface, said surface is exposed to a flux of UV radiationhaving a wavelength between about 110-180 nm and a power of about 150watts for a time sufficient to remove adsorbed water molecules from saidsurface.
 4. The method of claim 3 wherein said UV radiation has awavelength between about 160-170 nm.
 5. The method of claim 2 whereinsaid precursor material is deposited onto said surface to a thickness ofbetween about 1-5μ.
 6. The method of claim 1 wherein said ions areselected from the group consisting of hydrogen, helium, neon, nitrogen,argon, methane, and carbon monoxide.
 7. The method of claim 6 whereinsaid precursor material is deposited onto said surface to a thickness ofbetween about 1-5μ.
 8. The method of claim 1 wherein, before condensingsaid precursor material onto said surface, said surface is exposed to aflux of UV radiation having a wavelength between about 110-180 nm and apower of about 150 watts for a time sufficient to remove adsorbed watermolecules from said surface.
 9. The method of claim 8 wherein, beforecondensing said precursor material onto said surface, said surface isexposed to a flux of UV radiation having a wavelength between about110-180 nm and a power of about 150 watts for a time sufficient toremove adsorbed water molecules from said surface.
 10. The method ofclaim 9 wherein said UV radiation has a wavelength between about 160-170nm.
 11. The method of claim 8 wherein said precursor material isdeposited onto said surface to a thickness of between about 1-5μ. 12.The method of claim 1 wherein said precursor material is deposited ontosaid surface to a thickness of between about 1-5μ.
 13. The method ofclaim 12 wherein said precursor material is deposited onto said surfaceto a thickness of between about 1-5μ.
 14. A method for sealing a porousanodized aluminum surface comprising:placing a component having ananodized aluminum surface in a vacuum chamber evacuated to a pressure ofabout 10⁻⁶ torr; condensing onto said surface a precursor material in anamount sufficient, upon ion bombardment, to form an inert, substantiallyimpermeable amorphous carbonaceous seal, wherein said precursor materialis selected from the group consisting of polyphenyl ether, polydimethylsiloxane, pentaphenyltrimethyl siloxane, and elcosyl napthalene;substantially simultaneously bombarding said anodized surface with anenergetic beam of ions at an energy between about 1 keV to about 1 Mevfor a time and at a linear energy of transfer sufficient to convert saidprecursor material into said inert, substantially impermeable amorphouscarbonaceous seal.
 15. The method of claim 14 wherein said ions areselected from the group consisting of relatively low mass gaseouselements and compounds.
 16. The method of claim 15 wherein said energyof ion bombardment is between about 20-100 keV.
 17. The method of claim16 wherein said ions are selected from the group consisting of hydrogen,helium, neon, nitrogen, argon, methane, and carbon monoxide.
 18. Themethod of claim 16 wherein, before condensing said precursor materialonto said surface, said surface is exposed to a flux of UV radiationhaving a wavelength between about 110-180 nm and a power of about 150watts for a time sufficient to remove adsorbed water molecules from saidsurface.
 19. The method of claim 18 wherein said UV radiation has awavelength between about 160-170 nm.
 20. The method of claim 15 whereinsaid ions are selected from the group consisting of hydrogen, helium,neon, nitrogen, argon, methane, and carbon monoxide.
 21. The method ofclaim 15 wherein, before condensing said precursor material onto saidsurface, said surface is exposed to a flux of UV radiation having awavelength between about 110-180 nm and a power of about 150 watts for atime sufficient to remove adsorbed water molecules from said surface.22. The method of claim 21 wherein said UV radiation has a wavelengthbetween about 160-170 nm.
 23. The method of claim 22 wherein said UVradiation has a wavelength between about 160-170 nm.
 24. The method ofclaim 22 wherein said precursor material is deposited onto said surfaceto a thickness of between about 1-5μ.
 25. The method of claim 15 whereinsaid precursor material is deposited onto said surface to a thickness ofbetween about 1-5μ.
 26. The method of claim 14 wherein said energy ofion bombardment is between about 20-100 keV.
 27. A method for sealing aporous anodized aluminum surface comprising:placing a component havingan anodized aluminum surface in a vacuum chamber evacuated to a pressureof about 10⁻⁶ torr; condensing onto said surface a precursor material inan amount sufficient, upon ion bombardment, to form an inert,substantially impermeable amorphous carbonaceous seal, wherein saidprecursor material is selected from the group consisting of polyphenylether, polydimethyl siloxane, pentaphenyltrimethyl siloxane, and elcosylnapthalene; substantially simultaneously bombarding said anodizedsurface with an energetic beam of ions at an energy between about 1 keVto about 1 Mev for a time and at a linear energy of transfer sufficientto convert said precursor material into said inert, substantiallyimpermeable amorphous carbonaceous seal, wherein said ions are selectedfrom the group consisting of hydrogen, helium, neon, nitrogen, argon,methane, and carbon monoxide.